Liquid phase diffusion welding method for metallic machine part and metallic machine part

ABSTRACT

A liquid phase diffusion bonding method for a metal machine part superior in the quality of the joint and the productivity enabling the bonding time to be shortened, achieving homogenization of the bonding structure and improving the tensile strength, fatigue strength, and joint quality and reliability. This liquid phase diffusion bonding method of a metal machine part is characterized interposing an amorphous alloy foil for liquid phase diffusion bonding at bevel faces of metal materials, performing primary bonding by melt bonding said amorphous alloy foil and said metal material by resistance welding to form a joint, then performing secondary bonding by liquid phase diffusion bonding by reheating said joint to at least the melting point of said amorphous alloy foil, then holding it there to complete the solidification process of said joint.

TECHNICAL FIELD

The present invention relates to a method of production of a metalmachine part and to such a metal machine part, more particularly relatesto liquid phase diffusion bonding method of a metal machine part usedfor an auto part etc. and to such a metal machine part.

BACKGROUND ART

In the past, as the methods of bonding metal materials with each other,welding methods had mainly been used. In recent years, however, use ofthe liquid phase diffusion bonding method as new industrial bondingtechnology for replacing this has been spreading.

The liquid phase diffusion bonding method is the technology ofinterposing between bonding faces of bonded materials, that is, thebevel faces, an amorphous alloy foil with a melting point lower than thebonded materials, specifically a multimetal alloy foil having at least50% of its crystal structure amorphous, containing an element having theability to form a bonded joint through a diffusion-limited isothermalsolidification process, for example, B or P, and comprising a basematerial of Ni or Fe, then heating and holding the joint at atemperature of at least the melting point of this amorphous alloy foilso as to form a joint by an isothermal solidification process.

This liquid phase diffusion bonding method enables bonding with lowerheat input compared with ordinary welding methods, so is characterizedin that almost no residual stress of the weld occurs along with heatexpansion and contraction and no excessive buildup of the weld such aswith welding methods occurs, so the bond surface is smooth and aprecision bonded joint can be formed.

In particular, since liquid phase diffusion bonding is facial bonding,the bonding time does not depend on the area of the bonding faces and isconstant. Further, the bonding is completed in a relatively short time.From these viewpoints, this is bonding technology of a conceptcompletely different from the conventional welding methods. Therefore,there is the advantage that if a joint can be held for a predeterminedtime at a temperature of at least the melting point of the amorphousalloy foil inserted between the bevel faces of the bonded materials,bonding between the surfaces can be realized without having to selectthe bevel shape.

The applicant has already proposed a method for producing a metalmachine part provided with a pipeline inside it using this liquid phasediffusion bonding method in Japanese Unexamined Patent Publication(Kokai) No. 2003-181651 and Japanese Unexamined Patent Publication(Kokai) No. 2001-321963.

However, the liquid phase diffusion bonding disclosed in these patentpublications enables the bonding time to be made a relatively shorttime, but the isothermal solidification proceeds limited by diffusion.In so far as this is the case, in order for diffusion atoms in theamorphous alloy foil to diffuse and disperse in an amount sufficient forraising the melting point of the joint, when using an amorphous alloyfoil of a thickness of 10 μm, it is necessary to isothermally hold thefoil at about 900 to 1300° C., corresponding to a temperature of atleast the melting point of the alloy foil, for at least about 60seconds.

By making the amorphous alloy foil used for the liquid phase diffusionbonding thinner, the bonding time can be shortened to a certain extent,but the effect of the precision of working of the bevel faces of thebonded materials on bonding defects and the deterioration of the jointstrength becomes greater, so there are also limits to the reduction inthickness of the alloy foil. In actuality, the concentration of thediffusion atoms is raised in order to lower the melting point of thebonding foil or the chemical composition of the bonded materials isrelied upon to induce melting of the parent material at the time ofbonding. As a result, the actual thickness of the bonding alloy foilquite often exceeds 50 μm.

Further, even if raising the pressing stress in liquid phase diffusionbonding, while it is possible to shorten the bonding time to a certainextent, raising the pressing stress makes the bonded materials moresusceptible to buckling deformation, so there are limits to increasingthe pressing stress.

Accordingly, in the methods for producing metal machine parts usingliquid phase diffusion bonding disclosed in Japanese Unexamined PatentPublication (Kokai) No. 2003-181651 and Japanese Unexamined PatentPublication (Kokai) No. 2001-321963, improving the productivity of themetal machine parts and reducing the production costs by shortening thebonding time while maintaining the joint quality in liquid phasediffusion bonding has become an issue in the industry.

On the other hand, electrical resistance welding is known as a bondingtechnique frequently used for bonding metal machine parts in the past.

Electrical resistance welding is a method of utilizing the heat ofresistance produced by passing a current through metal, giving a largecurrent to make the bevels of the bonded materials instantaneously melt,and pressing the bevels to form a bonded joint.

For example, when melt bonding a thermocouple to a measured object formeasurement of its temperature, bonding steel plate to a frame member ofan automobile, and in other cases where the relative bonding area issmall and a high bonding strength is not required, spot welding,projection welding, upset welding, and other electrical resistancewelding methods are frequently used as simplified bonding methods.Conversely, when bonding large bevels with relatively large bondingareas, flash pad welding and continuous electrical resistance weldingable to apply a large current and a high pressing force areutilized—such as for seam welding of metal pipe.

However, when using these resistance welding methods to produce metalmachine parts, sometimes fluctuations in the bonding conditions willcause for example bonding defects due to residual oxide-based inclusionsat the bonds or insufficient welding current will cause so-called “coldwelding” or welding defects due to insufficient melting. Further, thepressing at the time of bonding causes large deformation to occur, finecracks occur at the welded parts, and the bevel ends remain unbonded,which become causes of a reduction in joint performance, particularlyfatigue strength. In particular, when at least one bonded material is acylindrical metal material, the drop in the joint fatigue strength tendsto become remarkable. As measures against this, in the past, forexample, changes in the material design or post-processing for improvingthe shape of the weld was required and there were problems such as thelimitations on the freedom of the joint design, increase of costs, etc.

In addition to this, in resistance welding, sometimes the weld width isextremely narrow and bevel deformation occurs, so quality assurance bynondestructive testing was difficult. Due to this and other reasons,improvement of the bonding quality in resistance welding in bondingjoints where reliability is particularly required is an issue inindustrial technology.

Further, Japanese Unexamined Patent Publication (Kokai) No. 11-90619,Japanese Unexamined Patent Publication (Kokai) No. 11-90620, andJapanese Unexamined Patent Publication (Kokai) No. 11-90621 disclose amethod and apparatus for bonding metal members making joint use ofliquid phase diffusion bonding and conduction type resistance welding inthe bonding of Al-based cylinder head members and Fe-based valve seats,but in each case the technique is just simple primary bonding resistancewelding with the interposition of a brazing material.

That is, no isothermal solidification diffusion treatment is beingperformed to convert the incomplete isothermal solidification structuresof the resistance welds occurring in primary bonding in the methodsdisclosed in the above patent publications to liquid phase diffusionbonding structures, so it is difficult to sufficient raise the qualityof the bonds.

Further, in the above art, a brazing material is pressed out untilbecoming extremely thin. The steps up to this are treated as part of theproduction process. Therefore, homogenization of the bond structure isnot being considered. Further, these disclosed art are technologies forforming joints for heterogeneous bonding of nonferrous metals such asAl. There is no description at all regarding the bonding of ferrousmaterials, in particular iron base materials. Of course, ordinarywelding can be used for iron base materials and use of ordinary weldingtechnology is difficult for bonding heterogeneous joints. Therefore,technology for bonding iron base materials is not described in the abovepatent publications.

DISCLOSURE OF THE INVENTION

The present invention considers the problems harbored by the above priorart and has as its object the provision of a liquid phase diffusionbonding method for a metal machine part enabling the bonding time to beshortened compared with the conventional liquid phase diffusion bondingmethod, achieving homogenization of the bonding structure andimprovement of the tensile strength, fatigue strength, and other aspectsof joint quality and reliability compared with the conventionalresistance welding methods, and superior in the quality of the joint andthe productivity and of a metal machine part assembled using the same.

The present invention was made to solve the above problems and has asits gist the following:

(1) A liquid phase diffusion bonding method of a metal machine partcharacterized by interposing an amorphous alloy foil for liquid phasediffusion bonding between bevel faces of metal materials, performingprimary bonding by melt bonding said amorphous alloy foil and said metalmaterial by resistance welding to form a joint, then performingsecondary bonding by liquid phase diffusion bonding by reheating saidjoint to at least the melting point of said amorphous alloy foil, thenholding it there to complete an isothermal solidification process ofsaid joint.

(2) A liquid phase diffusion bonding method of a metal machine part asset forth in (1), characterized in that the holding time after saidreheating is at least 30 seconds.

(3) A liquid phase diffusion bonding method of a metal machine part asset forth in (1) or (2), characterized in that the composition of saidamorphous alloy foil is Ni or Fe as a base and, as diffusion atoms, oneor more of B, P, and C in amounts of 0.1 to 20.0 at % and further V in0.1 to 10.0 at %.

(4) A liquid phase diffusion bonding method of a metal machine part asset forth in any one of (1) to (3), characterized in that saidresistance welding is one type of welding method from among conductionheating type spot welding, projection welding, upset welding, and flashpad welding and in that a time of melt bonding said amorphous alloy foiland said metal material by said resistance welding is not more than 10seconds.

(5) A liquid phase diffusion bonding method of a metal machine part asset forth in any one of (1) to (4), characterized in that an amount ofcurrent in said resistance welding is 100 to 100,000 A/mm².

(6) A liquid phase diffusion bonding method of a metal machine part asset forth in any one of (1) to (5), characterized in that a pressingforce in melt bonding of said amorphous alloy foil and said metalmaterial by said resistance welding is 10 to 1,000 MPa.

(7) A liquid phase diffusion bonding method of a metal machine part asset forth in any one of (1) to (6), characterized in that a thickness ina pressing direction of an incomplete isothermally solidificationstructure in a cross-sectional structure of a joint formed by saidresistance welding is on an average not more than 10 μm.

(8) A liquid phase diffusion bonding method of a metal machine part asset forth in any one of (1) to (7), characterized in that a jointefficiency of a joint formed by said resistance welding is 0.5 to 2.0where, the “joint efficiency” is the ratio of the area of the bevelfaces of the metal materials to the area of the joint after melt bondingthe amorphous alloy foil and metal materials

(9) A liquid phase diffusion bonding method of a metal machine part asset forth in any one of (1) to (8), characterized by cooling said jointafter the end of an isothermal solidification process by a cooling rateof 0.1 to 50° C./sec to control the joint structure.

(10) A metal machine part comprised of a joint formed by liquid phasediffusion bonding of metal materials, said metal machine partcharacterized in that a maximum grain size of prior γ phase in a metalstructure of the metal machine part as bonded is not more than 500 μm.

(11) A liquid phase diffusion bonding method of a metal machine part asset forth in any one of (1)-(9), characterized in that at least one ofsaid metal materials is a cylindrical metal material and in that aV-bevel is formed at an end of said cylindrical metal material so that,when bringing the end of said cylindrical metal material into abutmentwith the surface of another metal material for primary bonding, an innersurface bevel height A and an outer surface bevel height B of saidcylindrical metal material with respect to the abutting contact pointand a distance C from said abutting contact point to the outercircumference satisfy the following relation (1):0.2≦B/A≦1 and C/t≦0.5  (1)

where A is an inner surface bevel height of said cylindrical metalmaterial, B is an outer surface bevel height of said cylindrical metalmaterial, C is a distance from an abutting contact point of thecylindrical metal material to the outer circumference, and t is thethickness of the cylindrical metal material.

(12) A liquid phase diffusion bonding method of a metal machine part asset forth in (11), characterized in that a maximum residual height ofthe bevel ends after said primary bonding is not more than three timesthe thickness of said amorphous alloy foil.

(13) A liquid phase diffusion bonding method of a metal machine part asset forth in (11) or (12), characterized in that a joint efficiencyafter said primary bonding is at least 0.8.

(14) A liquid phase diffusion bonding method of a metal machine part asset forth in any one of (11) to (13), characterized in that a maximumresidual height of the bevel ends after said secondary bonding is notmore than 70 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of the relationship between the thickness of an alloylayer formed by the melting and solidification of an amorphous alloyfoil for liquid phase diffusion bonding and the holding time until theisothermal solidification of the alloy layer ends.

FIG. 2 is a view of the relationship between the isothermalsolidification holding time and bonded joint strength in secondarybonding (liquid phase diffusion bonding) of the method of the presentinvention.

FIG. 3 is a perspective view of an embodiment in the case of buttbonding a cylindrical metal material and another metal material.

FIG. 4 is a cross-sectional view of a bevel at the time of primarybonding of a cylindrical metal material and another metal material.

FIG. 5 is a cross-sectional view of a bevel running along a center axisof a steel pipe and vertical to the bonding faces before bonding metalmachine parts.

FIG. 6 is a view of the relationship between a ratio (B/A) of the outersurface bevel height B to the inner surface bevel height A of acylindrical metal material before primary bonding and the maximumresidual height of the bevel ends after the primary bonding.

FIG. 7 is a view of an embodiment in the case of welding a rectangularpipe and a branch pipe to produce a metal machine part.

FIG. 8 is a cross-sectional perspective view of a rectangular pipe fromthe inside pipeline axial direction at the time of assembly in FIG. 7.

FIG. 9 is a cross-sectional perspective view of an embodiment in thecase of butt bonding a metal material and a hollow metal material.

FIG. 10 is a cross-sectional view of a bevel running along a center axisof a hollow metal material and vertical to the bonding faces beforebonding of the metal machine part.

BEST MODE FOR WORKING THE INVENTION

Below, details of the present invention will be explained.

The method of the present invention uses metal materials as the bondedmaterials, brings them into abutment while interposing an amorphousalloy foil for liquid phase diffusion bonding between the bevel facesformed at the end of a metal material, and performs primary bonding bymelt bonding said amorphous alloy foil and said metal materials byresistance welding to form a joint.

In this primary bonding, for example, a resistance welding systemarranging at the bonded materials electrodes for supplying a weldingcurrent to the bevel faces (abutting faces) of the bonded materials toheat and melt them and using a stress applying mechanism to apply thestress required for press bonding between the bevel faces, for example,an oil pressure actuated Instron type tension/compression system, isused.

In this primary bonding, the welding heat input of the resistancewelding melts the bevel faces of the bonded materials and the liquidphase diffusion bonding alloy foil. The oxides upset by the pressingstress and produced at the time of heating and melting and theinclusions present at the bevel faces are discharged from the bondingfaces together with the molten metal.

Further, in the primary bonding, the amorphous alloy foil for liquidphase diffusion bonding inserted between the bevel faces of the bondedmaterials has a lower melting point compared with the ferrous materialsof the bonded materials. An amorphous alloy foil having a structure inwhich at least 50% of the volume of the foil is amorphous is used.

By interposing between the bevel faces of the bonded materials a liquidphase diffusion bonding alloy foil with a melting point of about 900 to1200° C. or lower than the bonded materials and melt bonding byresistance welding, there are the effects that the liquid phasediffusion bonding alloy foil is uniformly melted between the bevel facesand, at the same time, the oxides produced at the time of heating andmelting and the inclusions which had been left at the bevel faces aredischarged from the bonding faces together with the molten metal.

Note that the amorphous alloy foil for liquid phase diffusion bonding inthe present invention is comprised of Ni or Fe as a base material,contains as diffusion atoms one or more of B, P, and C in amounts of 0.1to 20.0 at % each, and further contains V having the action of loweringthe melting point of the oxides produced between the bonding faces atthe time of primary bonding in an amount of 0.1 to 10.0 at %.

The B, P, and C in the liquid phase diffusion bonding alloy foil areelements required as diffusion elements for realizing the isothermalsolidification required for achieving the liquid phase diffusion bondingof the secondary bonding or for lowering the melting point from thebonded members. To sufficiently obtain this action, they must becontained in amounts of at least 0.1 at %, but if excessively added,coarse borides, metal compounds, or carbides will be produced in thecrystal grains and the strength of the bond will fall, so the upperlimit is preferably made 20.0 at %.

The V in the liquid phase diffusion bonding alloy foil has the action ofreacting instantaneously with the oxide produced between the bevel facesat the time of the resistance welding of the primary bonding or theresidual oxide (Fe₂O₃) and changing them to a low melting point complexoxide (V₂O₅—Fe₂O₃, melting point: not more than about 800° C.) and givesthe effect of melting and discharging the low melting point complexoxide along with the molten metal by the pressing stress at the time ofthe resistance welding and reducing the oxide-based inclusions of thebond. To sufficiently obtain this action and effect, V is preferablyincluded in an amount of at least 0.1 at %. On the other hand, ifexcessively adding V in an amount over 10.0 at %, the number of theV-based oxide particles will increase and the residual oxide willconversely increase. Further, the melting point of the liquid phasediffusion bonding alloy will be raised and liquid phase diffusionbonding of the secondary bonding will be made difficult. Therefore, theupper limit is preferably made 10.0 at %.

Further, the resistance welding able to be used as the primary bondingin the present invention may be any welding method from among conductionheating type spot welding, projection welding, upset welding, and flashpad welding. Normally, spot welding, projection welding, and upsetwelding are suited for bonding in cases where the bonding area isrelatively small and a high bonding strength is not required, whileflash pad welding enables a large current and high pressing force to beapplied, so is suited to the case of bonding bevels of relatively largebonding areas. The selection of the resistance welding method does notparticularly have to be limited. It is preferable to suitably select itin accordance with the features of the different welding methods, therequired properties of the bonded joint, the welding conditions, etc.and to make the welding time not more than 10 seconds so as to improveproductivity.

Further, in order for the welding heat input for the resistance weldingin the primary bonding to melt the amorphous alloy for the liquid phasediffusion bonding between one bevel face and the other bevel face in ashort time, the current density must be made at least 100 A/mm². On theother hand, if the current density is raised excessively, the moltenmetal of the amorphous alloy foil will become disturbed and distributingit uniformly over the bevel faces by a predetermined thickness willbecome difficult, so the upper limit has to be made not more than100,000 A/mm². Therefore, it is preferable to make the current densityof the resistance welding from 100 to 100,000 A/mm².

Further, the pressing stress of the resistance welding in the primarybonding has to be at least 10 MPa in order to reduce the thickness ofthe bonding alloy layer formed by melting and solidifying the amorphousalloy foil for liquid phase diffusion bonding between the bevel facesand shorten the bonding time of the liquid phase diffusion bonding ofthe secondary bonding. On the other hand, if the pressing stress isexcessively high, deformation of the bonded joint occurs, so it has tobe made not more than 1000 MPa. Therefore, the pressing stress of theresistance welding is preferably made 10 to 1,000 MPa. Note that theextent of deformation of the bonded joint differs depending on theYoung's modulus of the bonded materials at the welding temperature, sothe upper limit of the pressing stress is preferably one adjusted by theproperties of the bonded materials.

Further, the joint efficiency of the joint formed by the resistancewelding at the primary bonding (area of bevel faces of ferrousmetal/area of joint after melt bonding amorphous alloy foil and ferrousmetal) has to be at least 0.5 taking into consideration the jointconstraint effect after bonding due to the shape of the bevels and tosecure a static tensile strength of the joint of a tensile strength ofat least the level of the parent materials. Further, the high pressingstress at the time of resistance welding causes the joint to swell. As aresult, the joint area becomes broader than the sectional area of theparent materials. Considering this, to obtain good joint properties, theupper limit is preferably made 2.0.

By melt bonding by the primary bonding shown above the amorphous alloyfoil for liquid phase diffusion bonding inserted between the bevel facesof the bonded materials in a short time, it is possible to form abonding alloy layer of an extremely thin thickness formed by the meltingand solidification of the amorphous alloy foil. The inventors conductedexperiments by which they confirmed from the results of examination ofthe joint cross-sectional structure by optical microscope that thethickness of the bonding alloy layer comprised of a structure formed bythe melting and solidification of the amorphous alloy foil obtained bythe primary bonding was a maximum of not more than 7 μm and was anaverage of not more than 3 μm.

The bonding alloy layer formed by the melting and solidification of theextremely thin amorphous alloy foil for liquid phase diffusion bondingin this way substantially finishes being isothermal solidification inthe subsequent liquid phase diffusion bonding of the secondary bondingby holding it at a temperature of at least the melting point of theamorphous alloy foil for about 15 seconds. If holding it for about 30seconds, when using ordinary carbon steel as the bonded materials, itwas confirmed by estimation by diffusion formula and experiments that acomplete isothermal solidification structure is obtained.

FIG. 1 is a view of the relationship between the thickness of the alloylayer formed by the melting and solidification of the amorphous alloyfoil for liquid phase diffusion bonding (in the case of the method ofthe present invention, the alloy layer after the primary bonding and inthe case of the conventional method, the alloy layer after pressbonding) and the holding time until the isothermal solidification of thealloy layer ends (holding time until incomplete isothermalsolidification structure can no longer be observed).

In the conventional liquid phase diffusion bonding method, the thicknessof the alloy layer formed by the melting and solidification of theamorphous alloy foil for liquid phase diffusion bonding can be reducedto a certain extent by increasing the pressing force, but an increase inthe pressing force causes joint deformation to occur, so as shown inFIAG. 1, making the thickness of the alloy layer thinner to not morethan 10 μm is difficult. The holding time until the isothermalsolidification of the liquid phase diffusion bonding ended had to be atleast 100 seconds. If making the isothermal solidification holding timein the prior art method less than 100 seconds, the problem arose that anincomplete isothermal solidification structure of the amorphous alloyfoil end up remaining and the strength, toughness, and other propertiesof the joint ended up becoming remarkably lower compared with the parentmaterials.

As opposed to this, with the method of the present invention, primarybonding (resistance welding) enables the average thickness of thebonding alloy layer produced by the melting and solidification of theamorphous alloy foil for liquid phase diffusion bonding to be reduced tonot more than 7 μm, while the following secondary bonding (liquid phasediffusion bonding) enables the holding time until the isothermalsolidification of the liquid phase diffusion bonding ends (untilincomplete isothermal solidification structure of bonding alloy layercompletely disappears) to be shortened to not more than 30 seconds.Experiments by the inventors, as shown in the figures, confirmed thatthe average thickness of the bonding alloy layer can be reduced to 3 μmby the primary bonding (resistance welding). In this case, it ispossible to expect the isothermal solidification to be completed(incomplete isothermal solidification structure of bonding alloy layerto completely disappear) by a holding time of 15 seconds by thesecondary bonding (liquid phase diffusion bonding). Due to the above, bythe method of the present invention, it is possible to greatly shortenthe bonding time and to expect an improvement in productivity whilemaintaining the joint quality at least equal to the conventional liquidphase diffusion bonding.

FIG. 2 is a view of the relationship between the isothermalsolidification holding time in the secondary bonding (liquid phasediffusion bonding) of the method of the present invention and the bondedjoint strength.

Note that the bonded joint strength is shown by the ratio of the tensilestrength of the bonded joint to the tensile strength of the parentmaterials in the case of conducting a tensile test in the directionpulling the joint from the bonding faces. If the value is 1, this meansthat the parent material breaks, while if less than 1, it means that thejoint breaks.

In actual bonding, the thickness of the bonding alloy layer produced bythe melting and solidification of the amorphous alloy foil for liquidphase diffusion bonding formed between the bevel faces by the primarybonding (resistance welding) of the present invention varies dependingon the position of the bevel faces, but from FIG. 2, by making theisothermal solidification holding time in the secondary bonding (liquidphase diffusion bonding) at least 30 seconds, a tensile test of thejoint results in the parent material breaking and therefore a good jointstrength of at least the tensile strength of the parent material isobtained.

In the method of the present invention, based on the above experimentalfindings, to secure a joint strength of at least equal to theconventional liquid phase diffusion bonding method, it is preferable tomake the isothermal solidification holding time of the secondary bonding(liquid phase diffusion bonding) at least 30 seconds.

Note that the isothermal solidification holding time of the secondarybonding (liquid phase diffusion bonding) can give a predetermined jointstrength stably along with its increase, but if the isothermalsolidification holding time is excessively increased, the old γ-crystalgrain size of the metal structure of the joint will become coarser andthe toughness of the joint will fall, so the upper limit is preferablymade not more than 100 seconds.

In the present invention, after the secondary bonding, that is, afterthe end of the isothermal solidification of the liquid phase diffusionbonding, by controlling the cooling rate in accordance with the type ofthe steel of the bonded materials, a desired metal structure, forexample, if a carbon steel, ferrite+pearlite, ferrite, bainite,martensite, or other metal structure is obtained or, if austenite steel,a bonded joint is obtained with a good metal structure due to the actionof re-solution of precipitates and other inclusions occurring at thetime of bonding.

In the present invention, in order to secure the minimum low temperaturetransformation structure (bainite or martensite) ratio required forimproving the strength and toughness of the joint required for a machinepart for an automobile, it is preferable to make the cooling rate afterthe secondary bonding, that is, after the end of the isothermalsolidification of the liquid phase diffusion bonding, at least 0.1°C./sec. Excessive cooling becomes a cause of a reduction in thetoughness and ductility, so the upper limit of the cooling rate ispreferably made 50° C./sec. By controlling the cooling rate, it ispossible to form a sound, high affinity joint between ferrite steels,austenite steels, or ferrite steel and austenite steel.

Note that in the method of the present invention, after the abovecooling and for the purpose of thermally refining the metal structure,it is possible to reheat and perform quenching, tempering, quenching andtempering, and other heat treatment alone or repeated a plurality oftimes or in combination. In this case, the joint structure is made morehomogeneous and the effect of the present invention can be furtherraised.

Note that with a material with an aversion to retained austenite, deepcooling is also effective. Deformation due to ageing can be suppressed.

According to the embodiments of the present invention shown above, it ispossible to reduce the amount of deformation of a joint compared withthe conventional resistance welding methods alone. Further, whenassembling metal machine parts etc., the invention can be applied tomachine parts of shapes unable to be processed even when utilizingboring, lathing, cutting, and other machining, machine parts includingheterogeneous welded joints of difficult to weld materials hard tocombine, and machine parts where the material cost would risetremendously due to cutting and therefore an improvement of productivityand further a reduction of costs and other effects can be simultaneouslyachieved. Further, according to the embodiments of the presentinvention, even in the case where small cracks occur in the bondingfaces after the primary bonding (resistance welding), the subsequentsecondary bonding (liquid phase diffusion bonding) causes the unmeltedamorphous alloy foil to further melt and flow into the cracks to therebyenable the fine cracks to be repaired. Further, the alloy layercomprised of the incomplete isothermal solidification structure ischanged to a complete isothermal solidification structure. Due to theseeffects, it is possible to obtain a joint higher in joint strength,fatigue strength, etc. and superior in quality compared with theconventional resistance welding methods.

Further, according to the embodiments of the invention, it is possibleto greatly shorten the isothermal solidification holding time of theamorphous alloy foil, that is, the time for holding the bonded joint ata reheating temperature of at least the melting point of the amorphousalloy foil, while maintaining a joint quality at least the same as theconventional liquid phase diffusion bonding method alone. As a result,the metal machine part comprised of a joint formed by liquid phasediffusion bonding with a ferrous metal material assembled according tothe method of the present invention can be given a maximum grain size ofthe prior y phase in the metal structure as bonded of a small size ofnot more than 500 μm and can be improved in toughness compared with ajoint obtained by the conventional liquid phase diffusion bonding method(where the crystal grain size is over a maximum grain size of 1 mm).

Further, at the time of secondary bonding (liquid phase diffusionbonding) in the embodiment of the present invention, since it ispossible not to apply pressure to the bonding faces as required at thetime of liquid phase diffusion bonding alone or to reduce the pressingforce, it becomes possible to obtain a good liquid phase diffusion bondwith just a simple pressing system. Therefore, according to the methodof the present invention, it becomes possible to realize liquid phasediffusion bonding free from weld defects and superior in bond quality ata low cost without requiring sophisticated pressing technology such aspressing the bonding faces uniformly at a high temperature.

Therefore, in metal machine parts assembled by the conventional liquidphase diffusion bonding, it becomes possible to simplify the QT andother heat treatment required for improving the joint toughness andimprove the productivity and reduce the production cost.

According to the embodiments of the invention explained above, it ispossible to produce bonded joints of metal materials at a higher qualityand higher productivity compared with the conventional bonding methodssuch as resistance welding methods alone and liquid phase diffusionbonding methods. However, as shown in FIG. 3, when at least one of themetal materials of the bonded materials is a cylindrical metal material11, when bringing its end into abutment with the other metal material 12and bonding them by primary bonding (resistance welding), the followingproblems are anticipated. Therefore, to stably achieve the effects ofthe present invention and stably improve the joint quality of thefatigue strength etc., it is preferable to use the embodiments explainedbelow.

That is, when bringing the cylindrical metal material 11 into abutmentwith the metal material 12 and bonding them by primary bonding(resistance welding), as shown in FIG. 4 (cross-sectional view of FIG.3), the pressing force 14 and thermal stress of the primary bondingcause the bevel parts of the cylindrical metal material 11 to flareoutward in the outer surface side direction 15 and therefore a bevel end13 of the outer surface side of the cylindrical metal material 11 afterthe primary bonding is easily left with a notch shaped groove. In thepresent invention, due to the secondary bonding (liquid phase diffusionbonding) performed after the end of the primary bonding, the unmeltedamorphous alloy foil is further melted and made to flow into theresidual notch shaped groove. However, if the maximum residual height 17of the bevel end becomes too large after the primary bonding, even withsubsequent secondary bonding, it becomes difficult to obtain a flatbond. The notch tip of the residual groove becomes a site of stressconcentration and becomes a cause of a reduction in the jointproperties, in particular the fatigue strength, so this is notpreferable.

In the present invention, to solve the above problems in the case ofprimary bonding (resistance welding) of at least one cylindrical metalmaterial and improve the joint properties more stably, the followingconditions are preferably defined.

FIG. 5 is a cross-sectional view of the bevel for explaining therelationship among the inner surface bevel height A and outer surfacebevel height B of the cylindrical metal material with respect to theabutting contact point, the distance C from the abutting contact pointto the outer circumference, and the thickness t of the cylindrical metalmaterial 11 at the time of bringing the end of the cylindrical metalmaterial 1 and the surface of the other metal material into abutment.Note that the cross-sectional direction of this figure runs along thecenter axis of the cylindrical metal material 11 and is vertical to thebonding faces.

In the present invention, to keep the bevel of the cylindrical metalmaterial 11 from flaring out in the outer surface side direction 15 dueto the pressing force 14 and thermal stress as shown in FIG. 4 at thetime of primary bonding of the cylindrical metal material 11 and toreduce the metal material 12 and the maximum residual height 17 of thebevel end after the primary bonding, it is preferable to make therelationship among the inner surface bevel height A and outer surfacebevel height B of the cylindrical metal material with respect to theabutting contact point 16, the distance C from the abutting contactpoint 16 to the outer circumference, and the thickness t of thecylindrical metal material 11 shown in FIG. 5 a suitable condition.

FIG. 6 shows the relationship between the ratio of the outer surfacebevel height B to the inner surface bevel height A of the cylindricalmetal material before primary bonding (in the state not pressed), thatis, B/A, and the maximum residual height 17 of the bevel end after theprimary bonding. Note that the ratio of the distance C from the abuttingcontact point 16 to the outer circumference to the thickness t of thecylindrical metal material 11, that is, C/t, was made 0.5.

Under conditions where the value of B/A is 0.2 to 1, it is learned thatthe maximum residual height after the primary bonding can besufficiently reduced. On the other hand, if the value of B/A becomesless than 0.2, the residual height of the inner surface bevel end of thecylindrical metal material 11 becomes larger. Further, if the value ofB/A exceeds 1, the residual height of the outer surface bevel end of thecylindrical metal material 11 will become larger. In both cases, themaximum residual height of the bevel end after primary bonding willbecome higher than 0.1 mm. In this case, it will become difficult tosufficiently reduce the residual groove of the joint even by the repairaction of the residual groove by the secondary bonding (liquid phasediffusion bonding) performed after the primary bonding, so this is notpreferred.

Further, FIG. 6 shows the results when the value of the ratio of thedistance C from the abutting contact point 16 to the outer circumferencewith respect to the thickness t of the cylindrical metal material 11,that is, C/t, is 0.5. Under conditions where the value of C/t becomessmaller than 0.5 or less, if a pressing force 14 is applied at the timeof the primary bonding as shown in FIG. 4, the bevel ends of thecylindrical metal material 11 will receive stress in the outer surfacedirection 15 and the outer surface bevel end will more easily deform, sothe maximum residual height of the bevel end after the primary bondingwill be reduced more. However, when the value of C/t becomes larger than0.5, if a pressing force 14 is applied at the time of the primarybonding, the inner surface bevel end of the cylindrical metal material11 will more easily deform. Even under conditions where the value of B/Ais 0.2 to 1, it will no longer be possible to reduce the maximumresidual height of the bevel end after the primary bonding to not morethan 0.1 mm, so this is not preferable.

Based on the above discovery, in the present invention, when at leastone of said metal materials is a cylindrical metal material, whenbringing the end of said cylindrical metal material into abutment withthe surface of the other metal material for primary bonding (resistancewelding), it is preferable to form a V-bevel at the end of thecylindrical metal material so that an inner surface bevel height A andan outer surface bevel height B of said cylindrical metal material withrespect to the abutting contact point and a distance C from saidabutting contact point to the outer circumference satisfy the followingrelation (1):0.2≦B/A≦1 and C/t≦0.5  (1)

Further, in the embodiments of the present invention, the maximumresidual height 17 of the bevel end of the cylindrical metal material 11after the primary bonding (resistance welding) shown in FIG. 4 ispreferably as low as possible so that the unbonded residual part of thebevel end can be sufficiently reduced and the fatigue strength of thejoint can be more stably improved by the melting and repair action ofunmelted amorphous alloy foil at the time of the secondary bonding(liquid phase diffusion bonding) performed after that. When exceedingthree times the thickness of the amorphous alloy foil, it becomesdifficult to more stably improve the fatigue strength of the joint evenby the melting and repair action of the unmelted amorphous alloy foil ofthe secondary bonding.

Therefore, in the above embodiments of the present invention,considering the melting and repair action and effect of the unmeltedamorphous alloy foil of the secondary bonding, the maximum residualheight of the bevel end after the primary bonding is preferably made notmore than 3 times the thickness of the amorphous alloy foil.

Further, in the embodiments of the present invention, when the pressingforce 14 or the welding current at the time of the primary bonding(resistance welding) is low or the conditions are otherwise unsuitableas shown in FIG. 4, even if the maximum residual height of the bevel endafter the primary bonding (resistance welding) of the cylindrical metalmaterial 11 is in the above suitable range, it will not be possible touniformly form a bonding alloy layer formed by the melting andsolidification of the amorphous alloy foil between the bevel faces afterthe primary bonding. Even if the area near the abutting contact point ispress bonded, the press bonding between the bevel faces will becomeinsufficient. Further, due to the effects of the residual stressoccurring due to the outer surface direction stress of the cylindricalmetal material 11 at the time of primary bonding (resistance welding),there is also the possibility that the bonding alloy layer will end uppeeling off before the secondary bonding.

To suppress these problems and form a uniform bonding alloy layerbetween the bevel faces after the primary bonding and form a bondingalloy layer with a good adhesion not peeling off before the secondarybonding, in the embodiments of the present invention, it is preferableto make the joint efficiency after the primary bonding at least 0.8.

Further, in the embodiments of the present invention, the maximumresidual height of the bevel ends after secondary bonding is preferablyas low as possible so as to further improve the joint fatigue strengthwithout using post-processing for improving the shape of the bevel ends.

In the embodiments of the present invention, due to the melting andrepair action of the unmelted amorphous alloy foil at the time of thesecondary bonding, the bonds can be made flat, but to further improvethe fatigue strength of the joints, it is preferable to make the maximumresidual height of the bevel ends after secondary bonding not more than70 μm.

EXAMPLES

The effects of the present invention will be explained by the followingexamples.

Example 1

Amorphous alloy foils for liquid phase diffusion bonding having thethree types of chemical compositions of the symbols A to C and meltingpoints shown in Table 1 and bonded materials comprised of ferrousmetals, Ni alloys, or Ti alloys having the chemical compositions of thesymbols a to f shown in Table 2 were used to produce metal machine partsunder the bonding conditions shown in Table 3 and Table 4.

The metal machine parts obtained were subjected to tensile tests in thedirection pulling away from the bonded surfaces and charpy impact testsof the bonds at 0° C. and were evaluated for joint strength and jointtoughness. Further, the amounts of deformation of the metal machineparts in the direction of application of bonding stress were measuredand the amounts of deformation evaluated. The results are shown in Table3 and Table 4.

Note that in Table 3 and Table 4, the evaluation of the joint strengthis shown by the ratio of the tensile strength of the bonded joint withrespect to the tensile strength of the parent material. If the value is1, this means that the parent material breaks, while if less than 1, itmeans that the joint breaks. Further, the evaluation of the jointtoughness is “good” when the absorbed energy at 0° C. is 21 J or moreand “poor” when it is less than 21 J.

Note that Nos. 2 to 8 shown in Table 3 are examples of production ofmetal machine parts by the following procedure. FIG. 7 and FIG. 8 areschematic views for explaining examples in the case of bonding a branchpipe 2 to a branch opening 4 of a rectangular pipe body 1 at the centerin the longitudinal direction of the internal pipeline 3 so as toproduce an automobile use metal machine part having a T-branch pipeinside. Note that FIG. 7 is a perspective view of a metal machine partfor an automobile, while FIG. 8 is a cross-sectional view running alonga center axis of the passage 2 and vertical to the center axis of theinternal pipeline 3.

As shown in FIG. 7, the end of the branch pipe 2 forming one bondingface was machined in advance to give a V-bevel having an angle of 45°.The bonding faces of the bevel 9 of the branch pipe 2 and the pipe body1 were made to abut against each other through a ring-shaped liquidphase diffusion bonding alloy foil 5, then electrodes 7 and 10 broughtinto close contact with the branch pipe 2 and the pipe body 1 were usedto run a DC current through the bevel parts. At the same time, apressing stress 24 was applied in the direction of 6. Note that thepressing stress was applied through a stress transmitting plate (notshown) operating by oil pressure from above the branch pipe 2. As aresult, the bevel 9 of the branch pipe 2 collapsed under the pressureand deformed to the same thickness as the thickness 8 of the branch pipe2. Further, the liquid phase diffusion bonding alloy foil 5 interposedbetween the bevels of the branch pipe 2 and the pipe body 1 formed analloy layer after melting once, then solidifying, but the bonding timewa extremely short, so the result was an incomplete isothermalsolidification structure with an average thickness of less than 3 μm,that is a so-called “brazed structure” where the diffusion-limitedisothermal solidification was not ended. Next, as secondary bonding, thebonded joint was raised to the reheating temperature of 150° C. by anelectric furnace having a high frequency induction heating coil andresistance heat generating element and held there for a predeterminedtime, whereby the diffusion-limited isothermal solidification of thebonding alloy layer formed in the primary bonding was ended, then thejoint was cooled.

Further, Nos. 1 and 9 shown in Table 3 are examples of production ofmetal machine parts by the following procedure.

As the bonded materials, two rods of diameters of 5 mm and lengths of 50mm were used. The bevel end faces were made completely I-shapes and wereground to bevel face roughnesses Rmax of not more than 10 μm. Alloyfoils for liquid phase diffusion bonding having diameters of 5 mm wereinterposed between these bevels, then the bonded materials were runthrough with DC current and simultaneously given a pressing stress forresistance welding as primary bonding to form joints. The absence of anyoffset in coaxiality of the rods was confirmed, then the now 100 mm longjoints were raised in temperature to the reheating temperature in anelectric furnace having a resistance heat generating element, then heldthere and then cooled as secondary bonding. There was no subsequent heattreatment at all.

Note that Nos. 10 and 11 shown in Table 4 are comparative examples whereno secondary bonding (liquid phase diffusion bonding) was performed whenproducing the above metal machine parts, while Nos. 12 and 13 arecomparative examples where no primary bonding (resistance welding) wasperformed when producing the above metal machine parts. Further, No. 14shown in Table 4 is a comparative example where primary bonding(resistance welding) and secondary bonding (liquid phase diffusionbonding) were performed when producing the above metal machine part, butthe bonding conditions were outside of the scope of the presentinvention.

From the results shown in Table 3, Nos. 1 to 9 of production of metalmachine parts under bonding conditions within the scope of the presentinvention by the bonding method of the present invention all had jointstrengths exceeding the tensile strengths of the parent materials andhad amounts of deformation in the direction of application of thebonding stress of not more than 5% or satisfactory in terms ofperformance in use as machine parts. Further, the holding times of theliquid phase diffusion bonding were short, so the maximum crystal grainsizes of the joints were fine sizes of not more than 500 μm and thejoint toughnesses were also excellent.

Nos. 10 to 14 shown in Table 4 are all comparative examples outside ofthe scope of the bonding conditions of the method of the presentinvention. Nos. 10 and 11 are comparative examples in the case of usingonly resistance welding. In this case, No. 10 had an amorphous alloyfoil for liquid phase diffusion bonding interposed, but its structurewas an incomplete isothermal solidification structure, that is, a brazedstructure, so the joint strength was lower than the parent materialstrength, the value of the evaluation was lower than the standard 1, andthe joint broke. In particular, No. 11 involved only resistance weldingwith no use of any amorphous alloy foil for liquid phase diffusionbonding, so inclusions and defects remained at the bonding interface andthe joint strength fell. With normal resistance welding, sometimes suchunstable joint strength results. These defects could not substantiallybe detected.

Further, Nos. 12 and 13 are comparative examples where bonding wasperformed by applying bonding stress of 5 MPa only at the liquid phasediffusion bonding. In the comparative example of No. 12, the holdingtime was a short 40 seconds. The crystal grains of the joint could bemade small, but the liquid phase diffusion bonding was not completed atall and therefore the joint strength fell. The comparative example ofNo. 13 had the holding time of the liquid phase diffusion bonding madelonger to make up for the elimination of addition of resistance welding.Therefore, the joint strength was improved, but the crystal grains ofthe metal structure became coarser. Therefore, the absorbed energy at 0°C. was less than 10 J and the toughness of the joint fell.

Further, No. 14 is an example where the liquid phase diffusion bondingtemperature in the secondary bonding was 820° C. or lower than anddeviating from the conditions of the present invention and the meltingpoint of the amorphous alloy foil for liquid phase diffusion bonding wasnot reached, so the brazed structure formed by the resistance weldingseparated into the borides and Ni base alloy and became brittle and thejoint ended up separating. TABLE 1 Chemical Composition of Bonding Foil(at %) Bonding Melting foil point symbol Base Si B P V (° C.) A Ni 3.5 811 1073 B Fe 2.5 12 8 1122 C Ni 0.8 15 7 942

TABLE 2 Main Chemical Composition of Bonded Materials (mass %) Bondedmaterial symbol Steel type C Si Mn Fe Cr Ni Mo Nb V N Al Ti a STPA28 0.10.3 0.5 bal. 9 — 1 0.05 0.2  0.04 — — b SCM440 0.4 0.25 0.7 bal. 1 — 0.2— — — — — c SUH11 0.5 1.8 0.2 bal. 9 — 0.5 — — — — — d SUH35 0.5 0.2 9bal. 21 — 4 — — 0.4  — — e INCONEL600 0.1 0.12 0.5 8.3 17 bal. — — — — —— f Ti-6Al-4V 0.05 — — 0.16 — — — — 4.24 — 6.21 Bal.

TABLE 3 Process Conditions and Joint Properties of Method of PresentInvention Primary bonding (provisional attachment for liquid phasediffusion bonding Secondary bonding by resistance welding) (liquid phasediffusion bonding) Total Liquid Maximum Current oxide Cooling phasecrystal value of length of Bonding Bonding rate diffusion grain Bondedresistance Applied bonding layer holding after bonding size of BondingEx. material welding pressure line width Joint time bonding temperaturejoint foil no. symbol (A/mm²) (MPa) (μm) (μm) efficiency (sec) ° C./s °C. (μm) symbol 1 a 180 50 0.2 4 0.8 31 5 1180 180 A 2 d 1200 50 0.1 41.3 38 5 1170 190 A 3 f 1500 80 0 6 1.2 90 5 980 250 C 4 b 1200 80 0.3 31.5 240 5 1190 300 B 5 c 25000 40 0.1 2 1.4 35 0.3 1240 240 A 6 e 4000030 0 5 1.8 1200 14 1200 480 B 7 a 1800 90 0 2 1.1 90 2 1150 140 B 8 b1200 30 0.3 6 1.9 81 1 1250 150 A 9 e 600 20 0.2 7 0.7 2400 10 1280 450A Evaluation of joint Joint Bonded strength/parent Ex. material materialJoint no. symbol strength toughness Deformation (%) Class 1 a 1.02 Good3.1 Inv. ex. 2 d 1.1 Good 3.7 Inv. ex. 3 f 1.03 Good 3.6 Inv. ex. 4 b1.2 Good 4.1 Inv. ex. 5 c 1.05 Good 3.9 Inv. ex. 6 e 1.02 Good 4.5 Inv.ex. 7 a 1.08 Good 3.6 Inv. ex. 8 b 1.4 Good 3.8 Inv. ex. 9 e 1.2 Good3.8 Inv. ex.Total oxide length of bonding line = total of lengths of oxide-basedinclusions present on substantially center line of bonding layer ofcross-sectional structure of bond when observed under opticalmicroscope/length of bonding layerBonding layer width = thickness of bonding layer parallel to directionof pressure at time of resistance welding of cross-sectional structureof bondJoint efficiency = (bevel area of bonded materials)/(area of jointportion after bonding where bonding metal foil is interposed)Maximum crystal grain size of joint = diameter of largest of old γ-grainsizes or ferrite grain sizes at bonding layer and heat affected zoneafter bondingJoint toughness = “good” when absorbed energy at 0° C. is 21 J or moreand “poor” when less than 21 J.Amount of deformation = amount of deformation in direction ofapplication of bonding stress

TABLE 4 Process Conditions and Joint Properties of Comparative ExamplesPrimary bonding (provisional attachment for liquid Secondary bondingphase diffusion bonding by resistance welding) (liquid phase diffusionbonding) Total Liquid Maximum Current oxide Cooling phase crystal valueof length of Bonding Bonding rate diffusion grain Bonded resistanceApplied bonding layer holding after bonding size of Bonding Ex. materialwelding pressure line width Joint time bonding temperature joint foilno. symbol (A/mm²) (MPa) (μm) (μm) efficiency (sec) ° C./s ° C. (μm)symbol 10 a 110 40 0.2 5 0.9 NA 11 b 180 30 18 (foil — 1.1 notinterposed) 12 c NA 40 0.5 1150 240 A 13 e 7200 0.2 1240 1200 B 14 d 56060 0.5 7 1.2 1500 0.7 820 450 A Evaluation of joint Joint Bondedstrength/parent Ex. material material Joint no. symbol strengthtoughness Deformation (%) Class 10 a 0.6 Good 5.1 Comp. ex. 11 b 0.4Good 5.6 Comp. ex. 12 c 0.3 Good 2.8 Comp. ex. 13 e 1.1 Poor 3.7 Comp.ex. 14 d 0 Good 5.3 Comp. ex.Total oxide length of bonding line = total of lengths of oxide-basedinclusions present on substantially center line of bonding layer ofcross-sectional structure of bond when observed under opticalmicroscope/length of bonding layerBonding layer width = thickness of bonding layer parallel to directionof pressure at time of resistance welding of cross-sectional structureof bondMaximum crystal grain size of joint = diameter of largest of old γ-grainsizes or ferrite grain sizes at bonding layer and heat affected zoneafter bondingJoint toughness = “good” when absorbed energy at 0° C. is 21 J or moreand “poor” when less than 21 J.Amount of deformation = amount of deformation in direction ofapplication of bonding stress

Example 2

Next, the same procedure as in the invention examples of Nos. 2 to 8shown in Table 3 of Example 1 was followed to bond branch pipes 2 andpipe bodies 1 shown in FIG. 8. At that time, as shown in Table 5, Table6, and Table 7, metal machine parts bonded under conditions changing thedimensions of the thickness t etc. of the branch pipes 2 of thecylindrical metal materials and the bevel conditions of the branch pipes2 (inner surface bevel height A, outer surface bevel height B, anddistance C from abutting contact point to outer circumference) at thetime of abutting were produced and were measured and evaluated for jointmechanical properties, in particular the fatigue strengths. Note thatthe chemical compositions of the amorphous alloy foils for liquid phasediffusion bonding and bonded materials used were the same as inExample 1. Further, the same procedure as in Example 1 was used forproduction except for the bonding conditions shown in Tables 5 and 6.

The metal machine parts obtained were subjected to tensile tests in thedirection pulling away from the bonding faces and fatigue impact testsand were evaluated for joint strengths and joint fatigue strengths. Theresults are shown in Table 6 and Table 7.

Note that in Table 6 and Table 7, the evaluation of the joint strengthis shown by the ratio of the tensile strength of the bonded joint withrespect to the tensile strength of the parent material. If the value is1, this means that the parent material breaks, while if less than 1, itmeans that the joint breaks. Further, the fatigue strengths of thejoints were measured and evaluated by subjecting the metal machine partsobtained to internal pressure fatigue tests and durability tests in astress range of 20 to 200 MPa and 10 million cycles (15 Hz) and judgingparts not cracking or breaking as “good” and parts cracking or breakingas “poor”.

Nos. 15 to 28 shown in Table 6 and Nos. 29 to 32 shown in Table 7 areall examples where the primary bonding (resistance welding) andsecondary bonding (liquid phase diffusion bonding) prescribed in thepresent invention are performed and where like the invention examplesshown in Example 1, the joint strength was greater than the strength ofthe parent materials and superior results were obtained in jointproperties compared with the conventional method.

Among the invention examples, Nos. 15 to 28 shown in Table 6 areinvention examples of production performed by bevel conditions of thecylindrical metal materials defined in the more preferable embodimentsof the present invention within the scope of the present invention,while Nos. 29 to 32 shown in Table 7 are invention examples ofproduction performed by conditions outside the more preferable range ofthe present invention.

Nos. 15 to 28 shown in Table 6 are examples where the relations amongthe inner surface bevel heights A and outer surface bevel heights B ofthe branch pipes 2 of the cylindrical metal materials, the distances Cfrom the abutting contact points to the outer circumferences, and thethicknesses t of the branch pipes 2 satisfy the more preferableconditions of the present invention, that is, 0.2≦B/A≦1 and C/t≦0.5, sonone of the bonded joints broke after the internal pressure fatiguetests and superior joint fatigue strengths were obtained. Further, sincethe holding times of the liquid phase diffusion bonding were short, themaximum crystal grain sizes of the joints were fine sizes of not morethan 500 μm and the joint toughnesses were also excellent. Among these,further, Nos. 16 to 22, 24, and 26 to 28 where the conditions of themaximum residual heights of the bevel ends after the primary bonding,the joint efficiencies after the primary bonding, and the residualheights of the bevel ends after secondary bonding were all in the morepreferable ranges of the present invention were further improved inresults in the joint fatigue tests compared with Nos. 15, 23, and 25deviating from one of these preferable conditions.

On the other hand, Nos. 29 to 32 of Table 7 are invention examples wherethe relations among the inner surface bevel heights A and outer surfacebevel heights B of the branch pipes 2, the distances C from the abuttingcontact points to the outer circumferences, and the thicknesses t of thebranch pipes 2 were outside the more preferred scope of the presentinvention.

Nos. 29 and 30 are examples where the ratios B/A of the heights of thebevel of the branch pipe 2 were larger than 1, that is, the outersurface bevel heights were higher than the inner surface sides. In thiscase, on top of the originally high outer surface side, the pipe flaresout slightly during the resistance welding. Due to this deformation, agroove easily remains at the outer surface side easily. When observingthe joint cross-section after the resistance welding, the residualheight of the bevel end became a large 100 μm or more and the jointefficiency also dropped. Further, No. 30 had an abutting contact pointpositioned closer to the inner surface side, that is, C/t>0.5, so thebevel at the inner surface side deformed preferentially and as a resultthe bevel end at the outer surface side largely remained.

No. 31 is an example where the ratio B/A of the heights of the bevel isin the scope of the invention, but when the position of the abuttingcontact point is C/t>0.5, in the same way as No. 30, deformationconcentrated at the inner surface side of the bevel, so the end of thebevel at the outer surface side remained and the residual height becamea large one of at least 100 μm.

As a result of the above, Nos. 29 to 31 cracked from the ends of thebevels at the outer surface sides in internal pressure fatigue tests andbroke at numbers of cycles below the predetermined number of cycles.

No. 32 is an example of a ratio B/A of heights of the bevel of not morethan 0.2, that is, where the height of the bevel at the inner surfaceside is at least five times the height of the bevel at the outer surfaceside. In this case, the bevel at the outer side deformed during theresistance welding and closed at the end, but much of the bevel at theinner surface side remained. The residual height of the bevel end becamea large one of at least 100 μm at the inner surface side. As a result,Comparative Example 32 cracked from the bevel end at the inner surfaceside in the internal pressure fatigue test and broke at a number ofcycles below the predetermined number of cycles. TABLE 5 Types of BranchPipes Branch pipe Test piece dimensions (mm) no. Outside dia. Insidedia. Thickness i 10 6 2 ii 26 17 4.5 iii 42 32 5 iv 50 38 6

TABLE 6 Process Conditions and Joint Properties of Method of PresentInvention Primary bonding (provisional attachment for liquid phasediffusion bonding by resistance welding) Current Maximum value ofresidual Test piece dimensions (mm) resistance Applied height ofDimensions of bevel part welding pressure bevel end Bonded A (inner B(outer material surface surface Joint Ex. no. symbol Pipe no. side)side) C B/A C/t (A/mm²) (MPa) (mm) efficiency 15 a i 0.07 0.06 1.00 0.860.50 500 45 0.109 0.85 16 c i 0.06 0.06 1.00 1.00 0.50 350 50 0.066 0.9317 e i 0.06 0.02 1.00 0.33 0.50 300 60 0.058 0.96 18 b i 0.06 0.02 0.700.33 0.35 350 50 0.044 0.99 19 a ii 0.04 0.02 2.25 0.50 0.50 250 600.074 0.95 20 c ii 0.05 0.02 2.00 0.40 0.44 250 60 0.035 0.94 21 b ii0.04 0.02 1.50 0.50 0.33 250 60 0.068 0.98 22 a iii 0.07 0.03 2.00 0.430.40 175 45 0.051 0.91 23 a iv 0.08 0.07 3.00 0.88 0.50 390 50 0.1130.79 24 b iv 0.05 0.03 2.00 0.60 0.33 250 60 0.072 0.94 25 d iv 0.100.02 2.00 0.20 0.33 255 60 0.098 0.87 26 e iv 0.08 0.04 3.00 0.50 0.50390 50 0.045 0.91 27 f iv 0.05 0.02 3.00 0.40 0.50 250 60 0.066 0.96 28c iv 0.05 0.01 2.00 0.20 0.33 250 60 0.044 0.98 Secondary bonding(liquid phase diffusion bonding) Joint evaluation Liquid phase MaximumJoint diffusion residual strength/ Internal Bonding Cooling rate bondingheight of parent pressure holding time after bonding temperature bevelend Bonding material fatique Ex. no. (sec) (° C./s) (° C.) (mm foilsymbol strength test Class 15 3600 10 1280 0.072 A 1.03 Good Inv. ex. 161800 2 1150 0.013 B 1.13 Good Inv. ex. 17 60 2 1250 0 C 1.15 Good Inv.ex. 18 360 50 1180 0 B 1.14 Good Inv. ex. 19 420 5 1200 0 A 1.1 GoodInv. ex. 20 2400 10 1170 0 A 1.19 Good Inv. ex. 21 90 5 1240 0 C 1.08Good Inv. ex. 22 1800 2 1150 0.011 C 1.07 Good Inv. ex. 23 1800 0.1 11600.081 A 1.03 Good Inv. ex. 24 400 5 1200 0.005 C 1.07 Good Inv. ex. 251200 5 1150 0.076 B 1.04 Good Inv. ex. 26 30 10 1250 0.023 A 1.05 GoodInv. ex. 27 600 10 1160 0 A 1.09 Good Inv. ex. 28 400 5 1200 0 B 1.16Good Inv. ex.Maximum residual height of bevel end = Maximum value of cylindrical axisdirection distance between bevel end and bonded surface after primarybonding or secondary bondingInternal pressure fatigue test = Test pieces passing durability testcomprising stress range of 20-200 MPa/10,000,000 repetitions (15 Hz)indicated as “good” and failing test as “poor”.

TABLE 7 Process Conditions and Joint Properties of Comparative ExamplesPrimary bonding (provisional attachment for liquid phase diffusionbonding by resistance welding) Test piece dimensions (mm) CurrentMaximum Dimensions of bevel part value residual Bonded A (inner B (outerof resistance Applied height of material surface surface weldingpressure bevel end Joint Ex. no. symbol Pipe no. side) side) C B/A C/t(A/mm²) (MPa) (mm) efficiency 29 a i 0.07 0.10 1.00 1.43 0.50 500 500.176 0.72 30 c ii 0.05 0.06 3.00 1.20 0.67 450 60 0.200 0.61 31 a iii0.07 0.07 3.00 1.00 0.60 300 65 0.156 0.73 32 a iv 0.08 0.01 2.50 0.130.42 380 50 0.320 0.67 Maximum residual height of bevel end = Maximumvalue of cylindrical axis direction distance between bevel end andbonded surface after primary bonding or secondary bonding Secondarybonding (liquid phase diffusion bonding) Liquid phase Maximum Jointevaluation Bonding Cooling rate diffusion bonding residual heightBonding Joint Internal Ex. holding time after bonding temperature ofbevel end foil strength/parent pressure no. (sec) (° C./s) (° C.) (mm)symbol material strength fatigue test Class 29 600 10 1250 0.123 A 1.03Poor Inv. ex. 30 360 5 1200 0.135 B 1 Poor Inv. ex. 31 400 5 1200 0.122C 1.06 Poor Inv. ex. 32 550 2 1200 0.25 B 1.06 Poor Inv. ex. Internalpressure fatigue test = Test pieces passing durability test comprisingstress range of 20-200 MPa/10,000,000 repetitions (15 Hz) indicated as“good” and failing test as “poor”.

Example 3

Next, an explanation will be given of an example of application of thebonding method of the present invention when producing hollow metalmachine parts such as various types of motor cam shafts fabricatedconventionally by casting, forging and cutting, etc. as shown in FIG. 9.

Liquid phase diffusion amorphous alloy foils having two types ofchemical compositions of the symbols A and B and melting points shown inTable 1 and bonded materials comprised of ferrous metals having thechemical compositions of the symbols “a” and “b” shown in Table 2 wereused to produce metal machine parts shown in FIG. 9 by the followingprocedure under the bonding conditions shown in Table 8.

That is, as shown in FIG. 9, the end of a hollow metal material 18forming one bonding face was machined in advance to give a V-bevelhaving an angle of 45°. The bonding faces of the bevel 19 of the hollowmetal material 18 and the metal material 20 were made to abut againsteach other through a ring-shaped liquid phase diffusion bonding alloyfoil 21, then electrodes 22 and 23 brought into close contact with thehollow metal material 18 and the metal material 20 were used to run a DCcurrent through the bevel 19 parts. At the same time, a pressing stress24 was applied. Note that the pressing stress 24 was applied through astress transmitting plate (not shown) operating by oil pressure fromabove the hollow metal material 18. As a result, the bevel 19 of thehollow metal material 18 collapsed under the pressure and deformed tothe same thickness as the thickness 25 of the hollow metal material 18.Further, the liquid phase diffusion bonding alloy foil 21 interposedbetween the bevels of the hollow metal material 18 and the metalmaterial 20 formed an alloy layer after melting once, then solidifying,but the bonding time was extremely short, so the result was anincomplete isothermal solidification structure with an average thicknessof less than 3 μm, that is a so-called “brazed structure” where thediffusion-limited isothermal solidification was not ended. Next, assecondary bonding, the bonded joint was raised to the reheatingtemperature described in Table 8 by an electric furnace having a highfrequency induction heating coil and resistance heat generating elementand held there for a predetermined time, whereby the diffusion-limitedisothermal solidification of the bonding alloy layer formed in theprimary bonding was ended, then the joint was cooled.

The metal machine parts obtained were subjected to tensile tests in thedirection pulling away from the bonding face and charpy impact tests atthe bonds at 0° C. and were evaluated for joint strengths and jointtoughnesses. Further, the amounts of deformation in the direction ofapplication of bonding stress of the metal machine parts were measuredand the amounts of deformation evaluated. The results are shown in Table8.

Note that in Table 8, the evaluation of the joint strength is shown bythe ratio of the tensile strength of the bonded joint with respect tothe tensile strength of the parent material. If the value is 1, thismeans that the parent material breaks, while if less than 1, it meansthat the joint breaks. Further, the evaluation of the joint toughness is“good” when the absorbed energy at 0° C. is 21J or more and “poor” whenit is less than 21 J.

From the results of Table 8, Example Nos. 33 to 35 of production ofmetal machine parts under bonding conditions in the scope of the presentinvention by the bonding method of the present invention all hadthicknesses of the bonding layers after primary bonding of an average ofnot more than 10 μm and joint strengths measured after secondary bondingof over the tensile strengths of the parent materials at all times.Further, the amounts of deformation in the direction of application ofbonding stress were not more than 5% or satisfactory in terms ofperformance in use as metal parts. Further, since the holding times ofthe liquid phase diffusion bonding were short, the maximum crystal grainsizes of the joints were fine sizes of not more than 500 μm and thejoint toughnesses were also good. TABLE 8 Process Conditions and JointProperties of Method of Present Invention Primary bonding (provisionalattachment for liquid phase diffusion bonding Secondary bonding byresistance welding) (liquid phase diffusion bonding) Total LiquidMaximum Current oxide Cooling phase crystal value of length of BondingBonding rate diffusion grain Bonded resistance Applied bonding layerholding after bonding size of Bonding Ex. material welding pressure linewidth Joint time bonding temperature joint foil no. symbol (A/mm²) (MPa)(μm) (μm) efficiency (sec) ° C./s ° C. (μm) symbol 33 a 180 100 0.4 50.9 60 5 1200 180 A 34 a 220 110 0.1 2 1.1 90 5 1200 200 B 35 b 180 900.3 3 1.1 60 5 1250 160 B Evaluation of joint Joint Bondedstrength/parent Ex. material material Joint no. symbol strengthtoughness Deformation (%) Class 33 a 1.09 Good 3.8 Inv. ex. 34 a 1.04Good 4.4 Inv. ex. 35 b 1.01 Good 4.1 Inv. ex.Total oxide length of bonding line = total of lengths of oxide-basedinclusions present on substantially center line of bonding layer ofcross-sectional structure of bond when observed under opticalmicroscope/length of bonding layerBonding layer width = thickness of bonding layer parallel to directionof pressure at time of resistance welding of cross-sectional structureof bondJoint efficiency = (bevel area of bonded materials)/(area of jointportion after bonding where bonding metal foil is interposed)Maximum crystal grain size of joint = diameter of largest of old γ-grainsizes or ferrite grain sizes at bonding layer and heat affected zoneafter bondingAmount of deformation = amount of deformation in direction ofapplication of bonding stress

INDUSTRIAL APPLICABILITY

As explained above, when using liquid phase diffusion bonding to form ajoint and produce a metal machine part, the present invention interposesan amorphous alloy foil for liquid phase diffusion bonding at the bevelsof metal materials, melt bonds the amorphous alloy foil by resistancewelding as primary bonding to provide an extremely thin bonding alloylayer formed by the melting and solidification of the amorphous alloyfoil, then provides an isothermal solidification process of liquid phasediffusion bonding at a reheating temperature of at least the meltingpoint of the amorphous alloy foil and thereby can give a joint withhomogeneity of structure and excellent tensile strength, toughness,fatigue strength, and other mechanical properties and with littledeformation. As a result, it is possible to produce metal machine partswith a high joint quality and reliability with a high productivity.Further, regarding the fatigue stress of a bonded joint with at leastone of its members comprised of a cylindrical metal material, which wasa problem in the conventional resistance welding methods, theinteraction between the primary bonding and secondary bonding of thepresent invention enables the occurrence of fine cracks in the weldedpart to be reduced and the unbonded residual amount of the bevel end tobe reduced and enables a joint superior in fatigue strength and a metalmachine part made by the same to be produced.

The present invention provides a completely new welding technique formetal machine parts enabling metal machine parts of shapes unable to beproduced in the past by ordinary machining, grinding, and boring andfurther low productivity, low material yield, high cost metal machineparts to be produced with a high productivity and low cost and cancontribute greatly to the improvement of functions and supply of metalmachine parts able to be achieved by the application of liquid phasediffusion bonding, In particular, in the production of camshafts andother hollow parts, shafts used for various types of motors, etc. whichused to be fabricated by casting, forging and cutting or metal machineparts using just a liquid phase diffusion bonding method in the past,application of the method of the present invention promises thereduction of the production cost, improvement of the productivity,improvement of the quality of the bond, and other effects. Thecontribution of the present invention to industry is therefore enormous.

1. A liquid phase diffusion bonding method of a metal machine partcharacterized by interposing an amorphous alloy foil for liquid phasediffusion bonding at bevel faces of metal materials, performing primarybonding by melt bonding said amorphous alloy foil and said metalmaterial by resistance welding to form a joint, then performingsecondary bonding by liquid phase diffusion bonding by reheating saidjoint to at least the melting point of said amorphous alloy foil, thenholding it there to complete an isothermal solidification process ofsaid joint.
 2. A liquid phase diffusion bonding method of a metalmachine part as set forth in claim 1, characterized in that the holdingtime after said reheating is at least 30 seconds.
 3. A liquid phasediffusion bonding method of a metal machine part as set forth in claim1, characterized in that the composition of said amorphous alloy foil isNi or Fe as a base and, as diffusion atoms, one or more of B, P, and Cin amounts of 0.1 to 20.0 at % and further V in 0.1 to 10.0 at %.
 4. Aliquid phase diffusion bonding method of a metal machine part as setforth in claim 1, characterized in that said resistance welding is onetype of welding method from among conduction heating type spot welding,projection welding, upset welding, and flash pad welding and in that atime of melt bonding said amorphous alloy foil and said metal materialby said resistance welding is not more than 10 seconds.
 5. A liquidphase diffusion bonding method of a metal machine part as set forth inclaim 1, characterized in that an amount of current in said resistancewelding is 100 to 100,000 A/mm².
 6. A liquid phase diffusion bondingmethod of a metal machine part as set forth in claim 1, characterized inthat a pressing force in melt bonding of said amorphous alloy foil andsaid metal material by said resistance welding is 10 to 1,000 MPa.
 7. Aliquid phase diffusion bonding method of a metal machine part as setforth in claim 1, characterized in that a thickness in a pressingdirection of an incomplete isothermally solidification structure in across-sectional structure of a joint formed by said resistance weldingis on an average not more than 10 μm.
 8. A liquid phase diffusionbonding method of a metal machine part as set forth in claim 1,characterized in that a joint efficiency of a joint formed by saidresistance welding is 0.5 to 2.0. where, the “joint efficiency” is theratio of the area of the bevel faces of the metal materials to the areaof the joint after melt bonding the amorphous alloy foil and metalmaterials
 9. A liquid phase diffusion bonding method of a metal machinepart as set forth in claim 1, characterized by cooling said joint afterthe end of an isothermal solidification process by a cooling rate of 0.1to 50° C./sec to control the joint structure.
 10. A metal machine partcomprised of a joint formed by liquid phase diffusion bonding of metalmaterials, said metal machine part characterized in that a maximum grainsize of prior γ phase in a metal structure of the metal machine part asbonded is not more than 500 μm.
 11. A liquid phase diffusion bondingmethod of a metal machine part as set forth in claim 1, characterized inthat at least one of said metal materials is a cylindrical metalmaterial and in that a V-bevel is formed at an end of said cylindricalmetal material so that, when bringing the end of said cylindrical metalmaterial into abutment with the surface of another metal material forprimary bonding, an inner surface bevel height A and an outer surfacebevel height B of said cylindrical metal material with respect to theabutting contact point and a distance C from said abutting contact pointto the outer circumference satisfy the following relation (1):0.2≦B/A≦1 and C/t≦0.5  (1) where A is an inner surface bevel height ofsaid cylindrical metal material, B is an outer surface bevel height ofsaid cylindrical metal material, C is a distance from an abuttingcontact point of the cylindrical metal material to the outercircumference, and t is the thickness of the cylindrical metal material.12. A liquid phase diffusion bonding method of a metal machine part asset forth in claim 11, characterized in that a maximum residual heightof the bevel ends after said primary bonding is not more than threetimes the thickness of said amorphous alloy foil.
 13. A liquid phasediffusion bonding method of a metal machine part as set forth in claim11, characterized in that a joint efficiency after said primary bondingis at least 0.8.
 14. A liquid phase diffusion bonding method of a metalmachine part as set forth in claim 11, characterized in that a maximumresidual height of the bevel ends after said secondary bonding is notmore than 70 μm.